hwc: Fix local conversion issue

* When the input layer is a video, 2020 layers can be converted to
  sRGB

* Restrict this change to secure videos only

Bug: 111881338
Test: UHD secure video in Dataspace::BT2020 can be played with HWC
      UHD non-secure video in Dataspace::BT2020 is played with GL
Change-Id: I17d13eda3a3915ed188326aa88073dc6e8fa896b
1 file changed